plugin/k8s_external/kubernetes: handle NS records (#3160)

* fix external ns records

* use k8s service name for ns record

* update test, add func comment

* expand nsAddrs() test cases

* support local ipv6 ip

* use less confusing pod ip in test

1 files changed, 6 insertions, 4 deletions

diff --git a/plugin/kubernetes/external.go b/plugin/kubernetes/external.go
index 4b1104460..42495a430 100644
--- a/plugin/kubernetes/external.go
+++ b/plugin/kubernetes/external.go
@@ -85,8 +85,10 @@ func (k *Kubernetes) External(state request.Request) ([]msg.Service, int) {
 
 // ExternalAddress returns the external service address(es) for the CoreDNS service.
 func (k *Kubernetes) ExternalAddress(state request.Request) []dns.RR {
-	// This is probably wrong, because of all the fallback behavior of k.nsAddr, i.e. can get
-	// an address that isn't reachable from outside the cluster.
-	rrs := []dns.RR{k.nsAddr()}
-	return rrs
+	// If CoreDNS is running inside the Kubernetes cluster: k.nsAddrs() will return the external IPs of the services
+	// targeting the CoreDNS Pod.
+	// If CoreDNS is running outside of the Kubernetes cluster: k.nsAddrs() will return the first non-loopback IP
+	// address seen on the local system it is running on. This could be the wrong answer if coredns is using the *bind*
+	// plugin to bind to a different IP address.
+	return k.nsAddrs(true, state.Zone)
 }